1. Field of the Invention
This invention relates to isotopic analysis associated with oil and gas drilling operations. Specifically, the present invention relates to an interpretive method utilizing newly developed mixing lines from derived mud gas isotope logging data to assess reservoir compartmentalization and hydrocarbon communication concomitant with identification of lithological seals, baffles and barriers.
2. Description of the Related Art
Laboratory analysis of gas samples obtained during a drilling operation may be employed to determine geochemical information associated with strikes of oil or gas deposits. The laboratory analysis may include the acquisition of compositional and isotopic data of sampled subsurface gases. This data is applied to traditional geochemical plots and templates. The interpretation of this data is used to provide geochemical information on the oil and gas provenance, how thermally mature the hydrocarbons are, whether subsurface post-generation effects were encountered during migration of the gaseous hydrocarbons from the source rock to a reservoir, and any problems or effects the hydrocarbons in the reservoir subsequently experienced.
Existing well sampling techniques use physical gas samples for compositional and isotopic laboratory analyses, obtained via wellheads, separators, down-hole logging tools (e.g., modular dynamic tester/repeat formation tester, etc.), canned cuttings, and/or sampled gases entrained in the mud system during drilling.
As discussed in co-pending U.S. patent application Ser. No. 10/845,743 ('743), there are several problems and issues not adequately addressed using standard mud gas chromatographic compositional analyses and interpretations. None of the existing techniques effectively detail or correlate geological information such as lithological hydrocarbon seals, baffles and barriers, good communication compartments, or gas diffusion and/or leakage into their interpretation. Compositional data can result in false positives and negatives where changes in operational conditions related to drilling variables such as increased rate of penetration or mud weight increases occur. '743 provides a far more advanced method which applies new interpretative techniques involving mud gas chromatographic compositional and isotopic analyses together with detailed drilling, geological and engineering information integration.
Within the improved interpretative techniques disclosed in '743 is the newly developed use of hydrocarbon mixing lines to determine or suggest good hydrocarbon communication compartments and zones. Mixing lines are identified on plots where hydrocarbon gas compositional and isotopic data are plotted. The mixing lines are defined by data points falling along a plotted trend line, suggesting a depth section in the well that is in good gas communication, and therefore representative of a compartment. Breaks in any of the mixing lines identify approximate depth locations at which lithological seals, baffles or other barriers to hydrocarbon communication may in fact be present. The depth range of each line may be considered to reflect or suggest an interval of good hydrocarbon communication. Furthermore, a number of seals, baffles and barriers are suggested defining these intervals, supporting the interpretation that these intervals may be likely to show localized hydrocarbon communication zones concomitant with potentially serious compartmentalization issues.
Thus, it would be a distinct advantage to have an interpretive method of analysis of mud gas samples utilizing chromatographic compositional and isotopic analysis together with an integrated geochemical, geological, and engineering interpretation applied to the data based on the use of these mixing lines. It is an object of the present invention to provide such an interpretative method specific to mud gas isotope logging.